Various polymer-modified asphalt compositions for paving and roofing applications and methods for preparing various polymer-modified asphalts are known. The degree to which a polymer improves an asphalt's properties depends on the compatibility of the polymer and the asphalt; e.g., a polymer that does not separate in a mixture of asphalt and polymer during storage. Highly compatible or compatibilized polymers are more effective in providing property improvements. An extensive range of additives have been used for the purpose of “crosslinking” polymers and asphalts, thereby rendering the mixture compatible. For example, sulfur is a well known crosslinking agent (U.S. Pat. Nos. 4,145,322 and 4,242,246). The addition of polymer to the asphalt first, followed by mixing, and then addition of the crosslinking agent is standard. In some instances, sulfur is added to the asphalt first, followed by the addition of polymer in some manner to the asphalt and sulfur (U.S. Pat. Nos. 5,672,642 and 7,037,961). In some instances, sulfur is added to one fraction of asphalt, polymer is added to a separate fraction of asphalt, and some portion of the two fractions are subsequently combined (U.S. patent application Ser. No. 12/242,579).
Asphalt modified with both a crosslinkable polymer and polyphosphoric acid and methods for preparing various modified asphalt compositions wherein the first modifier added to the asphalt is either polymer or phosphoric acid include: U.S. Pat. Nos. 5,880,185; 6,011,095; 7,495,045; and U.S. patent application Ser. No. 11/809,086. This order of addition is recognized in the art to result in cost-effective modified asphalt with acceptable rheological properties.